What is common knowledge in your field, but shocks outsiders? For over a century, from 1619 to ~1882, US states not the feds controlled international and interstate migration policy. A big reason, not the sole one, was slavery. Slave states saw federal deportation power as threatening to slavery.

apologies for the tangent - does California's "unfree labor"* during the gold rush play into this? we were late to the game but that gold was a quick path to power [*we relabelled slavery because there was no buying or selling 🤢]

Interesting. Hadn’t thought about from this angle.